(1) Field of the Invention
The present invention relates to a semiconductor PROM (Programmable Read Only Memory) device, more particularly to a semiconductor PROM device which uses a high potential voltage to write into information to the semiconductor PROM device, and which is used, for example, in an electronic computer.
(2) Description of the Prior Art
In a PROM device such as an EPROM (Erasable and Programmable Read Only Memory) device, the writing of information into each of the memory cells is effected by supplying a high potential voltage to each of the memory cells.
As illustrated in FIG. 1, the EPROM device comprises a plurality of memory cells C.sub.ij (i=1, 2, . . . , n; j=1, 2, . . . , m) disposed in a matrix of n rows and m columns. Each of the memory cells C.sub.ij is composed, for example, of a well known floating gate type field effect transistor which is connected between one of word lines WL.sub.1, WL.sub.2, . . . , WL.sub.n and one of bit lines (or column lines) BL.sub.1, BL.sub.2, . . . , BL.sub.m. Each of the bit lines BL.sub.1, BL.sub.2, . . . , BL.sub.m is connected through one of the gate transistors Q.sub.C1, Q.sub.C2, . . . , Q.sub.Cm respectively to a data input line D.sub.in. The data input line D.sub.in is connected through a switching transistor Q.sub.S to a data output line D.sub.out which is connected to a voltage source V.sub.CC of, for example, 5 V through a load transistor Q.sub.L.
In the EPROM device of FIG. 1, the writing of information, i.e., programming a memory cell such as C.sub.12 is effected by supplying a very high potential voltage, for example 20 V, to the selected word line WL.sub.1 and supplying a very high potential voltage, for example 20 V, to the data input line D.sub.in. Also, a bit address signal B is caused to be high and the inverted program signal PRG is caused to be low, so that the gate transistor Q.sub.C2 is turned on and the switching transistor Q.sub.S is turned off. Therefore, programming is achieved by simultaneously applying very high voltages to the drain and the control gate of the selected memory cell C.sub.12. Hot electrons are generated in the channel of the memory cell C.sub.12 and injected into the floating gate, resulting in an upward shift in threshold voltage V.sub.T of the memory cell transistor C.sub.12.
Reading information from the selected memory cell, for example, C.sub.12 is effected by supplying a high potential voltage, for example 5 V, to the selected word line WL.sub.1. Also, the bit address signal B.sub.2 is set to a high potential and the inverted program signal PRG is set to a high potential, so that the gate transistor Q.sub.C2 is turned on and the switching transistor Q.sub.S is turned on. If the floating gate of the memory cell C.sub.12 is charged, no current flows through the main current path of the memory cell transistor C.sub.12, so that the potential level of the data output line D.sub.out becomes high. If the floating gate of the memory cell C.sub.12 is not charged, a current flows from the positive voltage source V.sub.CC through the transistors Q.sub.L, Q.sub.S, Q.sub.C2 and through the memory cell transistor C.sub.12 to the ground. Therefore, the potential level of the data output line D.sub.out becomes low.
FIG. 2 illustrates a conventional word address decoder DEC having a decoder section and a high potential voltage supplying section HV connected between an output terminal A of the decoder section and the word line WL. The decoder section of the decoder DEC comprises a NOR gate NOR.sub.1 consisting of enhancement type transistors Q.sub.20, Q.sub.21, Q.sub.22, . . . and a depletion type load transistor Q.sub.1. The decoder section of the decoder DEC further comprises an output buffer circuit OB consisting of a depletion type load transistor Q.sub.3, a depletion type transistor Q.sub.5 and enhancement type transistors Q.sub.4, Q.sub.6. The high potential voltage supplying section HV comprises a depletion type transistor Q.sub.7 whose main current path is connected between the output terminal A of the decoder section of the decoder DEC and the word line WL, and, a first transistor Q.sub.8 of an enhancement type and a second transistors Q.sub.9 of a depletion type which are connected in series between the word line WL and a high potential voltage V.sub.PP of, for example, 25 V.
The circuit of FIG. 2 operates as follows. When the word line WL is selected, the potential levels of address signals a.sub.0, a.sub.1, a.sub.2, . . . are all low and the transistors Q.sub.20, Q.sub.21, Q.sub.22, . . . are all turned off so that the NOR gate NOR.sub.1 outputs a signal of high potential level. Therefore, the transistors Q.sub.4 and Q.sub.5 of the output buffer circuit OB are turned on and the transistor Q.sub.6 is turned off, so that the potential level of the output point A of the output buffer circuit OB becomes high.
When the writing of information is effected, the program signal PRG supplied to the gate electrode of the enhancement type transistor Q.sub.8 of the high potential voltage supplying section HV is set to a high potential. Therefore, the transistor Q.sub.8 is turned on and a current is supplied from the high potential voltage source V.sub.PP, whose potential voltage is, for example, 25 V, through the transistor Q.sub.8 and the depletion type transistor Q.sub.9 to the word line WL. In this condition, the potential level of an inverted signal PRG of the program signal is low, i.e., OV and the potential level of the output point A is high. Consequently, the transistor Q.sub.7, whose main current path is connected between the output point A and the selected word line WL, is turned off, because the gate threshold potential V.sub.th of the transistor Q.sub.7 is larger than -V.sub.CC and is smaller than 0 (i.e., 0&gt;V.sub.th &gt;-V.sub.CC, for example, V.sub.th =-V.sub.CC /2). Therefore, the potential level of the selected word line WL is nearly equal to V.sub.PP, i.e. 25 V, in the write-in mode.
When the reading of information is effected, the program signal PRG is caused to be low and the inverted signal PRG is caused to be high. Consequently, the transistor Q.sub.8 is turned off, the transistor Q.sub.7 is turned on, and a current flows from the voltage source V.sub.CC through the depletion type transistor Q.sub.5 of the output buffer circuit OB and the transistor Q.sub.7 to the selected word line WL. Therefore, the potential level of the selected word line WL is nearly equal to V.sub.CC, i.e. 5 V, in the read-out mode.
In the non-selected condition of the word line WL of FIG. 2, the potential level of at least one of the address signals a.sub.0, a.sub.1, a.sub.2, . . . is high and at least one of the transistors Q.sub.20, Q.sub.21, Q.sub.22, . . . is turned on. Therefore, the NOR gate NOR.sub.1 outputs a signal of low potential level, the transistor Q.sub.4 is turned off and the transistor Q.sub.6 is turned on, so that the potential of the output point A becomes low. In this condition, when the program signal PRG is high in the write-in mode, the transistor Q.sub.8 is turned on, and, the transistor Q.sub.7 is also turned on. This is because the gate signal PRG of the transistor Q.sub.7 is low, the potential level of the output point A is low, and the gate threshold potential V.sub.th of the transistor Q.sub.7 is larger than -V.sub.CC and smaller than 0, as mentioned above. Therefore, an idle current flows from the high potential voltage source V.sub.PP through the transistors Q.sub.8, Q.sub.9, Q.sub.7 and Q.sub.6 to the ground, and the potential of the word line WL becomes low. The transistor Q.sub.9 operates to pull up the word line WL to a high potential level when writing information. When the potential level of the word line WL becomes very high, the gate threshold potential V.sub.th rises due to the well known back gate bias effect. Therefore, the gate threshold potential V.sub.th of the transistor Q.sub.9 is set to a low negative value, for example -5 V, in order to keep the transistor Q.sub.9 in a turned on condition and pull up the word line WL to a very high potential level. Consequently, the above-mentioned idle current becomes large and the power consumption of the memory device increases.
The semiconductor memory device, for example the EPROM device, uses a large number of word address decoders and high potential voltage supplying sections. That is, the memory device uses the same number of word address decoders and high potential voltage supplying sections as there are word lines. Moreover, only one of the word lines can be selected at a time and all the other word lines are in non-selected condition. Therefore, the above-mentioned idle current becomes very large.
Two methods are employed in order to stop or decrease the idle current: (1) to increase the channel length of the transistor Q.sub.8 ; and (2) to increase the channel length of the transistor Q.sub.9. However, in both cases, the integration density of the semiconductor memory device is decreased. Therefore, in the conventional memory device, the gate threshold potential V.sub.th of the transistor Q.sub.9 is set to be a low negative value, but, the channel length of the transistors Q.sub.8 and Q.sub.9 is not increased.
Consequently, in the conventional semiconductor memory device, the idle current flowing in the non-selected decoders in the write-in or programming mode increases and the power consumption of the semiconductor memory device increases according to the number of the word address decoders.